In recent years, research into renewable energy resources that may replace fossil fuel has become an important issue. Among renewable energy resources into which research has been conducted, photogeneration, that is, solar cell-related techniques, have the advantages of the sun serving as an infinite energy source and a pollution-free pure energy source.
In contrast, high product prices and low generation efficiencies of solar cells act as disadvantages in large-scale power production using solar cells. To overcome these problems, research into new materials and structures of solar cells has progressed, and solar cells using compounds such as CuInS2 (CIS) and CuInGaSe2 (CIGS) have been suggested. Both CIS and CIGS have a high light absorptivity of about 5×105 cm−1, CIS has an energy band of about 1.5 eV, and CIGS has an energy band of about 1.36 eV. Also, since CIS and CIGS have low toxicities, it is appreciated that CIS and CIGS may be highly utilized in thin-film solar cells.
Furthermore, with regard to structure rather than material, solar cells using nanostructures having quantum effects, such as nanoparticles or nanowires, have been proposed instead of conventional thin film structures.
Korean Patent Publication No. 2010-97549 discloses a technique of forming zinc oxide (ZnO) nanowires on a substrate and forming a thin silicon layer on the ZnO nanowires. Also, U.S. Pat. No. 7,653,560 discloses a structure in which a lower electrode is formed using conductive nanowires and a semiconductor layer is stacked on the lower electrode.
In a solar cell using a nanostructure, a relatively large surface area can be secured and a diffusion distance with respect to source carriers is short. Consequently, charges can be smoothly transported and power loss caused by an internal resistance of the solar cell can be reduced.
However, in the solar cell using a nanostructure, light of different wavelength ranges cannot be absorbed. When a multilayered structure is used to solve this problem, a substantial multilayered structure cannot be formed due to fundamental limitations caused by an inter-material lattice constant and the nanostructure.
Accordingly, a structure of a solar cell capable of ensuring a large surface area and absorbing light having different wavelength ranges is needed.